This application claims the benefit of Japanese Application 2001-166,283, filed Jun. 1, 2001, the entirety of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an injection mold for casting semi-solidified Fe alloy that is in a solid-liquid coexistence state.
2. Description of Related Art
It is known to cast semi-solidified metal into a product by injection-molding method, such as rheocasting method or thixocasting method, wherein the semi-solidified metal is pressurized and injected into the mold cavity. Such injection-molding method proved to be highly advantageous in that, in contrast to conventional die-casting methods, the mold is subjected only to a relatively low level of thermal shock due to requirement for less preheating of the mold, besides lower casting temperature as well as less dissipation of solidification latent heat. For these grounds, the injection-molding method is generally regarded as a promising technology for casting metals with a relatively high melting point, e.g., Cu alloy and Fe alloy, which had been generally considered not very suitable for die-casting, primarily from economical viewpoint associated with a relatively short service life of the mold.
One may consider that the injection mold for semi-solidified metal can be formed of hard iron or steel material, such as hot die steel SKD61 (JIS G4404, ASTM H13), as in a die-cast mold for casting aluminum or the like light metal. As generally known in the art, however, iron or steel materials inclusive of SKD61 have a poor thermal conductivity of typically 40 W/(mxc2x7K) or less. Thus, when such materials are applied to an injection mold for casting metal, beside insufficient cooling capacity for the cast products or relatively long preheating time required for the mold, the following problems are likely to occur.
A) During gradual cooling and solidification of semi-solidified metal in the mold cavity, slurry tends to enter into clearances between knockout pins and surrounding holes, both provided for the mold, thereby forming undesirable flashes on the outer surface of the cast product, which must be removed to realize satisfactory product quality.
B) Plastic strains are accumulated in the mold due to large temperature gradient in the mold and repeated action of tensile and compressive stresses at the mold surface, and tend to cause premature crack formation in the mold. Moreover, severe stress concentration occurs at convex surface portions of the mold cavity having a small radius of curvature, so that hair cracks tend to be formed in the mold surface to shorten the life of the mold.
C) In the case of semi-solidified Fe alloy which comprises hypo-eutectic cast iron, for example, the poor cooling capacity of the mold leads to coarse graphite structure after annealing. In other words, it is difficult to obtain the desired fine graphite structure and sufficient mechanical strength of the cast products.
D) Upon injection of semi-solidified Fe alloy into the mold cavity, oxide film forming the outer surface of the alloy tends to enter into the mold cavity, thereby degrading the product quality.
It is therefore a primary object of the present invention to provide an improved injection mold for casting semi-solidified Fe alloy, which effectively eliminates the above-mentioned problems of the prior art.
It is more specific object of the present invention to provide an improved injection mold for casting semi-solidified Fe alloy, having excellent thermal conductivity and mechanical strength, and capable of effectively preventing entry of surface oxide film of the semi-solidified Fe alloy into the mold cavity.
The inventors conducted thorough research and investigations seeking a practical solution of the above-mentioned problems, and attained the following recognition.
First of all, copper alloys had been generally considered to be unsuitable as casting molds for high temperature materials, because copper alloy has strength inferior to iron or steel materials, despite higher thermal conductivity. Nevertheless, the inventors found that, since semi-solidified metal allows lower temperature of slurry upon injection molding, copper alloys having appropriately controlled composition to provide a sufficient hardness are still durable enough even when used as the material for the casting molds.
The inventors also found it effective to provide a scalping gate adjacent to the runner in communication with the mold cavity, and designed the scalping gate to have an opening diameter slightly smaller than that of the pressure chamber, so as to positively eliminate a surface oxide film of the semi-solidified Fe alloy as it is pressurized in the chamber and injected into the mold cavity.
Based on such recognition, an injection mold including a scalping gate was prepared from a copper alloy having a controlled composition, which had been adjusted to provide sufficient thermal conductivity and mechanical strength, to conduct trial injection molding of semi-solidified Fe alloy. As a result, it was found that considerable wear occurs at convex surface portions of the mold having a small radius of curvature near the opening of the scalping gate and within the mold cavity, indicating that the mold and the scalping gate require further improvement in terms of their durability for practical use.
The inventors then applied a cermet coating onto those surface portions of the mold and scalping gate, which are susceptible to wear, and conducted trial injection molding of semi-solidified Fe alloy. In this connection, the cermet coating was applied essentially as taught by U.S. Pat. No. 5,799,717, the disclosure of which is herein incorporated by reference. However, even by applying a cermet coating to the copper alloy base materials, it was found that the cermet coating tends to be peeled off during the actual injection molding, making it still difficult to achieve the desired durability of the mold and scalping gate.
The inventors analyzed the cause of the undesired peeling of the cermet coating and found that relatively acute convex portions of the mold and scalping gate are subjected to unexpectedly high thermal shock due to relatively high temperature of the semi-solidified Fe alloys as compared to Al or Al alloys to which U.S. Pat. No. 5,799,717 is directed, besides inclusion of solid components in the semi-solidified Fe alloys.
The inventors then thoroughly conducted experiments and investigations on measures effectively allowing formation of a stable cermet coating that can be maintained in firm adhesion to the base material and exhibiting excellent durability against high thermal shock upon injection molding of semi-solidified Fe alloy, to thereby provide an improved injection mold suitable for practical injection-molding of the semi-solidified Fe alloy. The present invention is based on a novel recognition that the stability of the cermet coating and, hence, the durability of the injection mold can be advantageously improved by applying a pre-coating of Ni alloy as an intermediate layer, before applying a cermet coating on the base material.
According to the present invention, there is provided an injection mold for semi-solidified Fe alloy, comprising a pair of mold members defining a mold cavity, and a scalping gate for removing a surface oxide film of the semi-solidified Fe alloy as it is pressurized in a pressure chamber in one of the mold members and injected into the mold cavity, the scalping gate being arranged between the pressure chamber and a runner in the other of the mold members, the runner being in communication with the mold cavity, the mold members and scalping gate each having a surface contacted by said semi-solidified Fe alloy during casting thereof:
The mold members and the scalping gate each comprise a copper alloy having a thermal conductivity of not less than 120 W/(mxc2x7K) and a hardness of not less than 180 HB, and
the mold members and the scalping gate each comprise a cermet layer consisting essentially of at least one member selected from a group consisting of Co, Cu, Cr and Ni. The cermet layer is formed by electro-spark deposition at least partially on the surface, via a Ni alloy intermediate layer which is also formed by electro-spark deposition.
It is additionally or alternatively preferred that the Ni alloy forming the intermediate layer has a composition consisting essentially of 30 to 50 mass %, in total, of at least one member selected from a group consisting of Cr, Fe, Mo and W, and the balance consisting of Ni and inevitable impurities.
It is additionally or alternatively preferred that the Ni alloy forming the intermediate layer has a film thickness within a range of 5 to 100 xcexcm, and an arithmetic mean surface roughness Ra within a range of 5 to 50 xcexcm.
It is additionally or alternatively preferred that the copper alloy has a composition consisting essentially of:
Ni: 1.0 to 2.0 mass %,
Co: 0.1 to 0.6 mass %,
Be: 0.1 to 0.3 mass %,
Mg: 0.2 to 0.7 mass %, and
Cu and inevitable impurities: the balance.
It is additionally or alternatively preferred that the cermet layer comprises one of WCxe2x80x94Co cermet, MoB2xe2x80x94Ni cermet and Cr3C2xe2x80x94Ni cermet.
It is additionally or alternatively preferred that the cermet layer has arithmetic mean surface roughness Ra within a range of 5 to 100 xcexcm.
It is additionally or alternatively preferred that the scalping gate comprises a coolant passage therein.